Vertical seismic profiling (VSP) is a technique that typically involves the creation of reflection images of the subsurface from seismic data acquired using an array of downhole geophones and a surface seismic source. A “reverse” VSP procedure is also known where the geophones are located at the surface and the seismic source (such as an operating drill bit) is located downhole. The usefulness of drilling noise as a source of seismic energy, however, depends in large part on the signal bandwidth and on how well one can estimate the acoustic signature of the radiated signal.
To use an operating drill bit as a seismic source, the continuous, chaotic signal generated at the bit must be converted to an equivalent impulse. Techniques described in previously published work in this area have typically relied on measurements made by accelerometers on the drill string to provide an estimate of the seismic signature of the drill bit. See, for instance, Staron, P., Gros, P., and Arens, G., 1985, UK Patent Application GB2173596A; Rector, J. W., Marion, B. P., and Widrow, B., 1988, Use of drill-bit energy as a downhole seismic source: 58nd Ann. Mtng., Soc. Expl. Geophys., Expanded Abstracts, 161-164; Rector, J. W., 1990, Utilization of drill bit vibrations as a downhole source: Ph.D. thesis, Stanford University; Rector, J. W., and Marion, B. P., 1991, The use of drill-bit energy as a downhole seismic source: Geophysics, 56, 628-634; and Rector, J. W., and Hardage, B. A., 1992, Radiation pattern and seismic waves generated by a working roller-cone drill bit: Geophysics, 57, 1319-1333, each of which is incorporated herein by reference. From this estimate of the seismic signature, an inverse filter is derived that is then applied to reduce the data to an estimate of the earth impulse response. To obtain a good estimate of the drill-bit signature from these accelerometers, one first has to remove the drill-string transfer function and unrelated noise from the accelerometer measurement.
A significant problem with this approach is that for deviated wells, friction between the borehole wall and the drill string may strongly attenuate the axial vibrations along the drill string and render the measurements made by the drill-string accelerometer effectively useless.
Another type of method is described in U.S. Pat. No. 5,148,407, to Haldorsen, J., Farmer, P., and Desler, J., 1992, entitled “Method for vertical seismic profiling”; in U.S. Pat. No. 4,922,362, to Miller, D., Haldorsen, J., and Kostov, C., 1990, entitled “Methods for deconvolution of unknown source signatures from unknown waveform data”; in Haldorsen, J.B.U., Miller, D. E., and Walsh, J., 1995, Walk-away VSP using drill noise as a source, Geophysics, 60, 978-997; and Haldorsen, J., Miller, D. Walsh, J., and Zoch, H.-J., 1992b, A multichannel approach to signature estimation and deconvolution for drill-bit imaging: 62nd Ann. Internat. Mtg., Soc. Expl. Geophysics, Expanded Abstracts, 181-184, each of which are incorporated herein by reference. This type of method makes use of the focusing capability of a large array of surface geophones both to obtain the drill-bit signature and to provide an optimal, multichannel deconvolution filter. However, in this type of seismic data acquisition scenario, the receivers are located in a very noisy environment (the earth's surface) and this introduces significant difficulty into the process of obtaining the drill-bit signature. In addition, the relatively large dimensions of the required surface geophone array (the array typically has a length that is approximately one or two times the depth of the drill noise source) have often limited the commercial viability of this type of “reverse” VSP procedure.
For these reasons, it would be of great benefit to be able to acquire and process seismic data obtained using a drill noise source using methods that do not suffer from one or more of the problems described above.